This invention relates to field-effect semiconductor devices, for example power MOSFETs, comprising a semiconductor body including a channel-accommodating region of a second conductivity type between source and rain regions of an opposite first conductivity type.
United States patent specification U.S. Pat. No. 5,438,215 describes several power MOSFETs having a body portion which separates the channel accommodating region from the drain region and which includes first and second additional regions of opposite (first and second) conductivity types. These regions of the body portion together provide a space charge region under the spread of a depletion layer from the channel-accommodating region to the drain region when a blocking voltage is present between the channel-accommodating region and the drain region in one mode of operation of the device. The body portion includes a drift region of the first conductivity type, for current flow of charge carriers of the first conductivity type to the drain region from a conduction channel in the channel-accommodating region in a conducting mode of operation of the device.
These MOSFETs of U.S. Pat. No. 5,438,215 are particular embodiments of the advantageous general device type that was disclosed in United States patent specification U.S. Pat. No. 4,754,310 (our ref: PHB32740). The first and second regions of the body portion serve together to carry the depletion layer from the drain region to the channel-accommodating region in an off state of the device. The second-type region of the body portion acts as a relief region. The first-type region of the body portion provides parallel current paths in a conducting state of the device. A significant improvement is obtained in the relationship between the on-resistance and breakdown voltage of the device, by so providing these first and second regions, instead of a single high-resistivity body portion as conventionally used. The whole contents of both U.S. Pat. Nos. 5,438,215 and 4,754,310 are hereby incorporated herein as reference material.
FIGS. 5 to 7 of U.S. Pat. No. 5,438,215 relate to MOSFETs in which both the body portion and the source and drain regions extend adjacent to a surface of the body. The additional relief regions of the second conductivity type are present in the drift region adjacent to the surface and extend longitudinally towards the drain region. They are spaced at their opposite ends from the channel-accommodating region and the drain region by interconnecting parts of the drift region. Additionally doped regions of the first conductivity type are present between these longitudinal relief regions of the second conductivity type. In the FIG. 7 embodiment, the width of these additionally doped regions of the first conductivity type increases towards the drain region to provide an increasing number of dopant atoms in the direction of the drain region, and thereby to allow a further increase in the breakdown field intensity.
United States patent specification U.S. Pat. No. 5,473,180 (our ref: PHN14508) also describes particular MOSFETs of the advantageous general device type, in which the drift region (termed the xe2x80x9cdrain extension regionxe2x80x9d) comprises a plurality of zones whose width may increase from the channel-accommodating region to the drain region, at the same surface of the body. Several advantageous geometries with increasing-width zones are disclosed in U.S. Pat. No. 5,473,180. The whole contents of U.S. Pat. No. 5,473,180 are hereby incorporated herein as reference material.
It is an aim of the present invention to provide a field-effect semiconductor device which is of the advantageous device type (wherein depleted opposite-conductivity-type regions of a body portion provide the space-charge region) and in which the spread of the depletion layer in the body portion is improved by the geometrical layout of the relief regions of the second conductivity type.
According to the present invention, there is provided a field-effect semiconductor device having the features set out in Claim 1.
Devices in accordance with the present invention have source and drain regions that are arranged concentrically so as to provide the body portion with an increase in its available layout area towards the drain region. Thus, the drain region is a surrounding region that extends (partially or entirely) around the body portion at the surface of the body. The relief regions are located radially in this body portion. The increased layout area towards the surrounding drain region readily permits the spacing between neighbouring relief regions of the second conductivity type to increase in the radial direction from the channel-accommodating region towards the surrounding drain region.
Their increasing spacing permits the radial relief regions to be formed with a doping concentration (of the second conductivity type) that is compatible with the different functions of these regions towards opposite ends of the body portion. At the end towards the channel-accommodating region, the radial relief regions of the second conductivity type are closely spaced and so they reinforce each other in their depletion of the drift region at this end. Thus, a moderate doping concentration of the second conductivity type for these relief regions can be high enough to give a strong electric-field push to spread the depletion layer away from the channel-accommodating region. Where they are widely spaced at the other end towards the drain region, this moderate doping concentration is low enough to permit full depletion of the relief regions when the full breakdown voltage is being supported by the spread of the depletion layer in the vicinity of the drain region. The adoption of these increasingly-spaced radially-arranged relief regions in this concentric arrangement permits a near-optimum field distribution to be achieved in the body portion, with a maximum field-relief adjacent to the channel-accommodating region.
The device may comprise a single concentric device cell in the semiconductor body. However its concentric/radial geometry can be adopted readily in a power device configuration, comprising a multicellular array of the body portions side-by-side, with the drain region extending as a mesh between the neighbouring body portions. The drain region may have a polygonal outline facing the body portion at the surface. The depletion layer can be spread into the internal corners of the polygonal outline by directing the relief regions radially towards the polygonal outline, for example towards its internal corners.
The drift region of the first conductivity type may also extend under the radial relief regions of the second conductivity type. In this case, the body region may further include an underlying region of the second conductivity type which underlies the drift region and which forms a p-n junction with the drift region. This underlying region may have a very low doping concentration of the second conductivity type, so permitting it to accommodate some of the depletion when the full breakdown voltage is being supported by the spread of the depletion layer in the vicinity of the drain region. Thus, the space charge region may extend into the radial relief regions, and/or the drift region and/or an adjacent part of the underlying region. The present invention permits the doping of the underlying region to be very low. Although it may be too low to give a strong push to the depletion layer away from the channel-accommodating region, an adequate push is provided instead by the high doping and closely spacing of the radial relief regions of the second conductivity type in the vicinity of the channel-accommodating region.